If you're as big a Google fan as I am, you can go to http://www.google.com/mac and your results will only be Macintosh related. Now there's no need to specify mac or macintosh in the query/input line.

[Editor's note: Google Mac has been listed here on the links page for a while, but I thought it was worth a mention in a hint -- you can eliminate many excess results if you're looking for Mac-specific information by using Google Mac.]

You'd probably have to edit the binary unless there's a hidden API to do it.
The 'strings' command will show the hard-coded data in the Safari binary:
strings /Applications/Safari.app/Contents/MacOS/Safari | grep -i google
Turns up 2 urls:
http://%@.google.com/%@?q=%@&ie=UTF-8&oe=UTF-8
http://%@.google.com/%@
If there are a lot of NULs after those strings you might be able to change the URL with a hex editor and not wreck the binary.